Transcript Theory Continental Drift

The
Theory of
Continental
Drift
Observations Leading To
and Evidences Supporting Theory
• You will…
• Fill in blanks/notes
• Fill in the observations to
support the theory
• Label the observations to
support the theory on your
diagram (by color coding)
• Label a diagram showing sea-floor
spreading and the magnetic
reversals recorded in the rock as
the sea-floor spread
Theory of Continental Drift
•Continental Drift is the theory
that the
CONTINENTS can DRIFT apart
from one another and have
done so in the past.
Observations
• Remember: A theory is a unifying
explanation for several observations that
has been supported by testing
• Alfred WEGENER developed his theory
that the continents can drift and have
done so in the past based on the following
observations:
Observations
• Shape/“Fit” of the continents
• The continents appear as if they could fit
together like jigsaw puzzle pieces!
1. Shape/“Fit” of Continents
PANGAEA
245 million years ago!
Observations
• Fossils of the same plant and animal
species found on continents separated
by oceans
• These organisms could
not have made it across
the Atlantic Ocean!
• The pattern of fossil
findings is hard to explain
unless at one point these
continents were not
separated by an ocean!
2. Same Fossils on
Different Continents
Question:
Would you
expect to find
the same
organisms on
different
continents?
For example,
would you
expect to find
the same
organisms in
Africa and
Antarctica?
Why or why
not?
Observations
• Same rocks on different continents
• Rocks that were
formed at the
same time under
the same
conditions with
the same
composition are
found on
continents
separated by
oceans
3. Same Rocks on
Different Continents
• This pattern
is hard to
explain
unless at one
point these
continents
were
together as
one!
Observations
• Same ancient climatic conditions
are found on several continents
• The climate of a continent depends upon its
location on the Earth relative to the sun.
• Ex. Equator = Hot; Pole = Cold
• Evidence of similar
climates on continents
in very different
locations on Earth is
hard to explain unless…
4. Same Ancient Climate on
Different Continents
• …. at one
point
these
continents
were
together
as one!
Critics
• Many scientists did not accept Wegner’s theory
of Continental Drift; they said the continents
cannot “plow through” the oceanic crust
because oceanic crust is much denser than
continental crust, and objects with less density
cannot push through objects with higher density.
So, what force could
move entire continents?
Evidences to Support Theory
1. SEA-FLOOR SPREADING
- the process by which
new oceanic crust forms
at the mid-ocean ridge.
Evidences to Support Theory
• Sea-floor
spreading was
discovered
when scientists
noticed the
pattern of
younger rocks
at the midocean ridges
and older rocks
farther away
from the ridges.
Evidences to Support Theory
When tectonic plates move
away from each other magma
rises to fill the gap, cools and
hardens creating new oceanic
crust.
Scientists finally knew why the
continents move!
The sea-floor spreads as plates
move away from each other!
Sea-floor Spreading
causes
Evidences to Support Theory
2. Magnetic Reversals are recorded in
the crust as the sea-floor spreads.
Earth’s Magnetic Field
• The Earth is a giant magnet!
• Every ~700 thousand years the
Earth’s magnetic North and South
poles change places.
• Will a magnetic reversal occur
sometime soon?
Evidences to Support Theory
THIS IS FURTHER PROOF THAT THE SEAFLOOR SPREADS, OR THAT NEW CRUST IS
CREATED OVER TIME
THERE IS A “TIME STAMP” OF EARTH’S
MAGNETIC FIELD IN THE ROCK
• Tiny grains of magnetic minerals in molten
rock will align with Earth’s magnetic field.
• When the rock cools and hardens the
Earth’s magnetic signature at that time is
recorded in the rock.
• You should have…
• Filled in the blanks/notes
• Filled in the observations to
support the theory
• Labeled the observations to
support the theory on your
diagram
• You now need to…
• Study Figures 5 & 6 on pp. 458459 of Sea-floor Spreading &
Magnetic Reversals
• Label the diagram showing seafloor spreading and the magnetic
reversals recorded in the rock as
the sea-floor spread
Sea-Floor Spreading
Magnetic Reversals
Pangaea
Sea-floor Spreading
Magnetic Reversals